Nowadays, with respect to ambient light detection for a room, for example, the use of an external ambient light sensor to control the switching on/off state of a compact fluorescent light (CFL) may be known. The ambient light sensor activates a controller of the CFL to either switch on or off the CFL.
The ambient light sensor comprises one or more photosensitive elements having an electrical property that changes upon incidence of light, together with circuitry for converting the changed electrical property into a signal. As an example, an image sensor may comprise a photodetector that generates a charge when radiation is incident upon it. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
The basic physical principles of operation of an ambient light sensor are similar to those of digital image sensors, such as complimentary metal oxide semiconductor (CMOS) or charge-coupled device (CCD) image sensors used in digital cameras, mobile telephones, webcams, and various other devices. In theory, an image sensor of that type could be used to provide information on the ambient light levels, however, these devices have many pixels and often complicated signal processing circuitry to decode color, defect correction, etc. meaning that they generally require a relatively larger amount of power to operate (50 mW typical). Also, as these sensors have a large number of pixels, it may be computationally expensive to process them. Finally, their sensors are usually color (e.g. Bayer pattern, as disclosed in U.S. Pat. No. 3,971,065) with different sensitivities for red, green, and blue, which may require additional processing to obtain only the brightness information. For these reasons, the use of a standard, mega-pixel type image sensor may not be appropriate for measuring ambient light levels.
An ambient light sensor is distinguished from a mega-pixel type image sensor by its relatively small number of pixels. The number of pixels may not yield sufficient image data for the reconstruction of an image, either for representation to a human or to a machine. An ambient light sensor may comprise a single photodetector, or a very small array of photodetectors, typically less than ten. Also, because of the relatively small number of pixels, the pixels of an ambient light sensor may typically be larger than those used in typical mega-pixel type image sensors.
The ambient light sensor detects the ambient light level, which corresponds to the level of (human) visible light. The visible light comprises atmospheric light levels from 400 nm to 700 nm, for example, and the light from the CFL. The detected level of visible light is compared with a reference threshold value to operate the controller to either switch on the CFL, or switch off the CFL, depending on the result of the comparison.
When the detection occurs during the night, and the CFL is switched off, for example, the atmospheric light levels may be zero or negligible, and there is typically no light from the CFL. As a result, there is zero or negligible visible light. Therefore, the detected level of visible light is below the reference threshold value, and the ambient light sensor operates the controller of the CFL to switch on the CFL.
However, to switch the CFL off at the right time, the ambient light sensor may distinguish between the atmospheric light and the light from the CFL itself. This problem is currently addressed by choosing a physical arrangement of components that seeks to reduce the interference of the two sources of light (atmospheric and CFL), for example, in a street light, the CFL will be directed in a downward direction, and the ambient light sensor will be directed in the opposite, upward direction, and also sometimes within a collar or other light shield to further isolate the ambient light sensor from the CFL. In an indoor environment, the shielding of ambient light sensors is even more difficult. However, as well as the additional structural complexities involved, the influence of the CFL light may not be perfectly isolated from the atmospheric light. Because of this, the thresholds that the ambient light sensor uses are sub-optimal, which may result in energy being wasted because CFLs are typically kept on for longer than needed during the day time.